1. Field of the Invention
The present invention generally relates to spray-deposited production of a product on a moving substrate and, more particularly, is concerned with use of a non-preheated substrate composed of low thermal conductivity material for reducing product porosity.
2. Description of the Prior Art
A commercial process for production of spray-deposited, shaped preforms in a wide range of alloys has been developed by Osprey Metals Ltd. of West Glamorgan, United Kingdom. The Osprey process, as it is generally known, is disclosed in detail in U.K. Pat. Nos. 1,379,261 and 1,472,939 and U.S. Pat. Nos. 3,826,301 and 3,909,921 and in publications entitled "The Osprey Preform Process" by R. W. Evans et al, Powder Metallurgy, Vol. 28, No. 1 (1985), pages 13-20 and "The Osprey Process for the Production of Spray-Deposited Roll, Disc, Tube and Billet Preforms" by A. G. Leatham et al, Modern Developments in Powder Metallurgy, Vols. 15-17 (1985), pages 157-173.
The Osprey process is essentially a rapid solidification technique for the direct conversion of liquid metal into shaped preforms by means of an integrated gas-atomizing/spray-depositing operation. In the Osprey process, a controlled stream of molten metal is poured into a gas-atomizing device where it is impacted by high-velocity jets of gas, usually nitrogen or argon. The resulting spray of metal particles is directed onto a "collector" where the hot particles re-coalesce to form a highly dense preform. The collector is fixed to a mechanism which is programmed to perform a sequence of movements within the spray, so that the desired preform shape can be generated. The preform can then be further processed, normally by hot-working, to form a semi-finished or finished product.
The Osprey process has also been proposed for producing strip or plate or spray-coated strip or plate, as disclosed in European Pat. Appln. No. 225,080. For producing these products, a substrate or collector, such as a flat substrate or an endless belt, is moved continuously through the spray to receive a deposit of uniform thickness across its width.
Heretofore, extensive porosity typically has been observed in a spray-deposited preform at the bottom thereof (its side in contact with the substrate or collector). This phenomenon, normally undesirable, is a particular problem in a thin gauge product, such as strip or tube, since the porous region may comprise a significant percentage of the product thickness. The porosity is thought to occur when the initial deposit layer is cooled too rapidly by the substrate, providing insufficient liquid to feed the inherent interstices between splattered droplets. In other words, when the semi-solid hot metal droplets hit the cool substrate, they transfer all of their heat and thus freeze before they can spread on the substrate and also before subsequent droplets arrive.
One approach of the prior art for reducing the porosity problem is preheating the substrate to minimize or reduce the rate of heat transfer from the initial deposit to the substrate so that rapid solidification of the droplets does not occur and some fraction liquid is always available to feed voids created during the spray deposition process. However, it is often difficult to effectively preheat a substrate in a commercial spray deposit system because of the cooling effects on the substrate of the high velocity re circulating atomizing gas.
Also, for wide width material, non-uniform heating of the substrate may occur which can lead to distortion of the substrate and the deposit and in extreme conditions may cause hot tearing of the deposit. In addition the probability of the deposit sticking or welding to the substrate increases with increasing preheat temperature. It is also important to note that other problems such as creep thermal fatigue, etc., of the substrate can arise if excessive preheat temperatures are required.
Therefore, a need exists for an alternative approach to elimination of the porosity problem particularly in thin gauge product produced by the above-described Osprey spray-deposition process.